Compositions or manufactured articles consisting of cross-linked polypropylene and non cross-linked polypropylene, obtained by cross-linking of polypropylene performed with organic peroxides and in the presence of cross-linking co-agents consisting of double or triple bonds, and optionally of foaming agents are known from the European Patent Application No. 73435.
According to this Patent Application, the preformed manufactured article to be cross-linked, is treated in an autoclave containing an aqueous solution of the peroxide and of the co-agent, at the cross-linking temperature. Alternatively, the manufactured article, obtained by extruding at temperature between 160.degree. and 180.degree. C. a mix comprising polypropylene, the peroxide and the co-agent, is separately cross-linked at temperatures between 180.degree. and 230.degree. C., without overpressure, or under reduced or high pressure.
These compositions show a poor homogeneousness and can be worked with difficulty. The foamed articles which are obtained do not have the flexibility requirements which are requested for many applications.